1. Field of the Invention
The present invention relates to a high voltage power semiconductor device that can control high current and particularly relates to a vertical high voltage semiconductor device that uses silicon carbide, which is a wide bandgap material, as a semiconductor and a manufacturing method of the vertical high voltage semiconductor device.
2. Description of the Related Art
Conventionally, single crystal silicon has been used as a material for high voltage semiconductor devices that control high current. At present, there are various types of power semiconductor devices and each is selectively used according to intended purpose. For example, bipolar transistors and insulated gate bipolar transistors (IGBT), which can handle high current density, cannot switch at high speeds and the application limit for bipolar transistors is a frequency of several kHz and for IGBTs, a frequency on the order of 20 kHz.
On the other hand, power MOSFETs, which cannot handle high current, can be used for switching at the high speeds up to a frequency of a few MHz. Nonetheless, market demand for power devices equipped to handle both high current and high speeds is strong and much effort has been poured into the improvement of IGBTs and power MOSFETs, to the point that at present, the performance of power devices has substantially reached the theoretical limit determined by the material.
FIG. 17 is a cross-sectional view of a conventional MOSFET. An n− drift layer 2 is formed stacked on an n+-type semiconductor substrate 1. In a surface layer of the n− drift layer 2, a p base layer 4 is selectively formed. In a surface layer of the p base layer 4, an n+ source layer 7 is selectively formed; and a gate electrode 8 is formed via a gate insulating film on the n− drift layer 2, the p base layer 4, and the n+ source layer 7. Recently, a MOSFET (hereinafter, superjunction MOSFET) that uses parallel pn layers of p-type regions and n-type regions of high impurity concentrations, in an alternating arrangement, as a drift layer has received much attention.
FIG. 18 is a cross-sectional view of a conventional silicon superjunction MOSFET. Further, FIG. 19 is a cross-sectional view of a silicon superjunction MOSFET by a conventional multi epitaxial growth method. FIG. 20 is a cross-sectional view of a silicon superjunction MOSFET by a conventional trench filling method.
These superjunction MOSFETs were presented in a research paper by Fujihira, et al in 1997 (refer to Fujihira, Tatsuhiko, “Theory of Semiconductor Superjunction Devices”, Jpn. J. Appl. Phys, Vol. 36, pp. 6254-6262, Part 1, No. 10, October 1997) and were produced in 1998 by Deboy, et al as “CoolMOS” (refer to Deboy, G., et al, “A new generation of high voltage MOSFETs breaks the limit line of silicon”, IEEE IEDM pp. 683-685, 1998). These superjunction MOSFETs are characterized in being formed in a pillar structure having a p layer in a vertical direction (in a direction of substrate depth) in an n− drift layer, enabling ON resistance to be dramatically improved without degrading breakdown voltage.
Further, material studies have been performed from the perspective of power semiconductor devices and as reported by Shenai, et al (refer to SHENAI, KRISHNA, et al, “Optimum Semiconductors for High-Power Electronics”, IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. 36, p. 1811-1823, 1989), recently, silicon carbide (SiC) in particular has gathered attention for use in devices having low ON voltage with excellent high-speed and temperature properties, as next generation power semiconductor devices. SiC is a very stable material chemically, has a wide bandgap of 3 eV, and can be used very stably as a semiconductor even at high temperatures. Further, the critical electric field strength of SiC is 10-fold that of silicon or higher. The material performance of SiC is likely to exceed the material performance limits of silicon and therefore, increased use of SiC for power semiconductors is greatly expected, especially for MOSFETs. In particular, there are high expectations related to the low ON-resistance of SiC and for a vertical SiC-MOSFET that has even lower ON-resistance, while maintaining high breakdown voltage.
The cross-sectional structure of a typical SiC-MOSFET is that depicted in FIG. 17 described above, similar to silicon. In the surface layer of the n− drift layer 2, the p base layer 4 is selectively formed. The n+ source layer 7 selectively formed in the surface layer of the p base layer 4 is formed; the gate electrode 8 is formed on the n− drift layer 2, the p base layer 4, and the n+ source layer 7, via the gate insulating film; and a drain electrode 11 is formed in the back surface of the semiconductor substrate 1.
A SiC-MOSFET thus formed is expected to be used as a device capable of high speed switching while having a low ON-resistance such as in a power conversion equipment like a motor control inverter or an uninterruptible power supply (UPS).
SiC is a wide bandgap semiconductor material and therefore, the critical electric field strength thereof is about 10 times higher than that of Si and the ON-resistance of SiC is expected to be sufficiently low. However, because the critical electric field strength of the semiconductor is about 10 times higher than that of Si, the electric field load on the oxide film becomes higher compared to that of a Si device especially when high voltage is applied.
Consequently, for silicon power devices, the critical electric field strength of Si is reached before a high electric field is applied to the oxide film and therefore, is not problematic. However, with a power device that uses SiC, there is concern that the oxide film will fail. For example, high electric field strength is applied to the gate insulating film (oxide film) of the SiC-MOSFET depicted in FIG. 17, potentially arising in a serious problem concerning the reliability of the SiC-MOSFET. This concerns not only SiC-MOSFETs but also SiC-IGBTs. Regarding this, for example, a document describes that care needs to be taken in terms of the electric field strength applied to the gate oxide film in the SiC-MOSFET (refer to U.S. Pat. No. 7,923,320).